Get ready for hotter summers and more flooding in the UK, say scientists

Weather-wise, the UK saw it all last year. The
coldest spring for 50 years, a sweltering summer heat wave and the
wettest winter since records began. Today, a new report examines
whether climate change is upping the odds of these events
occurring.

The collection of papers, published in a
bumper edition of journal Bulletin of the
American Meteorological Society, looks at 16 weather events that
took place last year across the world. From Colorado to Korea, the
scientists examine heatwaves, droughts, heavy rain and
storms.

Human fingerprints

Globally, there is
evidence for changes in some types of
extreme weather, and evidence for a human fingerprint in those
changes. But different types of event are affected
differently.

Climate change is greatly increasing the odds of
heatwaves worldwide, today’s report concludes. For storms, rainfall
and drought the picture is less clear, however. Big differences
between regions, natural variability in the climate and limited
data make detecting changes over time far more
difficult.

The science of disentangling human and natural
influences on our climate is known as attribution. Dr Peter Stott,
head of the climate change detection and attribution team at the
Met Office and an editor on the report, explained more in a
recent
guest blog for us:

“[The aim
is] to compare what actually happened with what might have happened
in a world without anthropogenic climate change.”

Understanding how our activities are changing
the risk of some types of extremes is important for making
decisions about how we can prepare for the future.

Hot summers

In summer 2013, western Europe experienced an
extreme heatwave. Average temperatures for the June to August
period sit just below those of 2003 – the hottest summer in Europe
for at least
500 years.

At the same time, the UK experienced its hottest
day since 2006 with temperatures of 33.5 degrees Celsius recorded
at Heathrow airport, the report notes.

A paper in
today’s collection concludes that while
natural variability played a role in warmer than average sea
surface temperatures in the North Atlantic, human activity played a
“substantial part” in bringing about the prolonged hot, dry
spell.

These conclusions build on previous research
suggesting human influence has at least
doubled the risk of a heatwave like the one
Europe experienced in 2003.

Professor Rowan Sutton, lead author on the new
paper, says:

“Climate change has
increased the odds of hot summers in the UK. Summer 2013 was a good
example, and Britain should expect more hot summers in the future,
although not every year.”

We should still expect the odd extreme cold
event despite average temperatures rising, the report notes. Spring
2013 in the UK was the coldest for more than half a century, for
example.

But climate change means we’ll see such events
less often. A paper in
today’s collection suggests warming has made
events like the UK’s 2013 cold spring 30 times less likely.
Globally, extreme cold events that used to occur every 20 years are
now happening every 35 years,
other research notes.

Wetter or drier?

Thanks to our notoriously complicated weather
systems in the UK, whether climate change will make our summers
wetter or drier is more difficult to unravel, Sutton
adds.

“Given the UK’s position
on the edge of the Atlantic, we think that – at present – natural
changes in winds and ocean currents play a bigger role in summer
rainfall patterns than emissions of greenhouse gases.”

But while it’s unclear from today’s research
what the change in the total amount of rainfall might be in the UK,
the risk of flooding is on the up. Sutton adds:

“[I]n a warmer world,
when summer rain does occur, it’s likely to be the heavy kind that
can cause floods.”

Recent research suggested climate change has
made the sort of heavy and prolonged rainfall we experienced in the
UK last year
25 percent more likely.

Total rainfall (mm) for January in southern
England from records going back to 1910. Source: “The recent storms
and floods in the UK” report (
Met Office)

Bigger picture

The report looks further afield at extreme
weather beyond the UK and Europe.

A collection of papers found human-caused
climate change has significantly increased the likelihood of the
record heatwave Australia experienced in 2013. As Professor David
Karoly from the University of Melbourne
puts it:

“If we were
climate detectives then Australia’s hottest year on record in 2013
wasn’t just a smudged fingerprint at the scene of the crime, it was
a clear and unequivocal handprint showing the impact of human
caused global warming.”

The impacts of man-made climate change were felt in
Australia during its hottest year on record in 2013. Source: UNSW,
P3, Helena Brusic.

Human influence on the ongoing California
drought is less clear. Overall, the scientists “did not find
conclusive evidence of human impacts”, the report notes.

One paper suggests the atmospheric pattern
scientists think is causing the current drought is becoming more
likely under climate change. But other research in today’s special
issue that uses a different approach isn’t so confident.

We’ve looked more closely at what the report
says about the California drought
here.

Scientists can’t say a single event is ever
solely caused by climate change, instead they talk about climate
change altering the odds of different types of extreme weather.
Even more so than in other aspects of climate science, it’s
important to clear about what question is being asked – and it’s
not a one-size fits all answer for every different type of
event.

The report also makes the point that while some
of today’s research doesn’t find a definitive link between some
types of extreme event and climate change, that doesn’t mean there
isn’t one. It might mean the signal from man-made warming hasn’t
yet risen above the noise of natural variability, but that could
change in the future.

Today’s new report adds a lot to the steadily
growing body of literature on climate change attribution. The
nuances of researching and communicating extreme weather show why a
reasoned and evidenced-based approach is a must when it comes to
reducing climate risk.